Permian
The Permian is a geologic period that extends from about 299.0 ± 0.8 Ma to 251.0 ± 0.4 Ma (million years before the present; ICS 2004). It is the last period of the Palaeozoic Era. Subdivisions The three primary subdivisions of the Permian Period are given below from youngest to oldest, and include faunal stages also from youngest to oldest. Additional age/stage equivalents or subdivisions are given in parentheses. Epoch and age refer to time, and equivalents series and stage refer to the rocks. Lopingian Epoch :Changhsingian Age (Djulfian/Ochoan/Dewey Lake/Zechstein) :Wuchiapingian Age (Dorashamian/Ochoan/Longtanian/Rustler/Salado/Castile/Zechstein) Guadalupian Epoch :Capitanian Age (Kazanian/Zechstein) :Wordian Age (Kazanian/Zechstein) :Roadian Age (Ufimian/Zechstein) Cisuralian Epoch :Kungurian Age (Irenian/Filippovian/Leonard/Rotliegendes) :Artinskian Age (Baigendzinian/Aktastinian/Rotliegendes) :Sakmarian Age (Sterlitamakian/Tastubian/Leonard/Wolfcamp/Rotliegendes) :Asselian Age (Krumaian/Uskalikian/Surenian/Wolfcamp/Rotliegendes) Oceans Sea levels in the Permian remained generally low, and near-shore environments were limited by the collection of almost all major landmasses into a single continent -- Pangaea. One continent, even a very large one, has less shoreline than six to eight smaller ones. This could have in part caused the widespread extinctions of marine species at the end of the period by severely reducing shallow coastal areas preferred by many marine organisms. Paleogeography During the Permian, all the Earth's major land masses except portions of East Asia were collected into a single supercontinent known as Pangea. Pangea straddled the equator and extended toward the poles, with a corresponding effect on ocean currents in the single great ocean ("Panthalassa", the "universal sea"), and the Paleo-Tethys Ocean, a large ocean that was between Asia and Gondwana. Cimmeria continent rifted away from Gondwana and drifting north to Laurasia, causing the Paleo-Tethys to shrink. A new ocean was growing on its southern end, the Tethys Ocean, an ocean that will dominate much of the Mesozoic Era. Large continental landmasses create climates with extreme variations of heat and cold ("continental climate") and monsoon conditions with highly seasonal rainfall patterns. Deserts seem to have been widespread on Pangea. Such dry conditions favored gymnosperms, plants with seeds enclosed in a protective cover, over plants such as ferns that disperse spores. The first modern trees (conifers, ginkgos and cycads) appeared in the Permian. Three general areas are especially noted for their Permian deposits: the Ural Mountains (where Perm itself is located), China, and the southwest of North America, where the Permian Basin in the U.S. state of Texas is so named because it has one of the thickest deposits of Permian rocks in the world. Life Permian marine deposits are rich in fossil mollusks, echinoderms, and brachiopods. Fossilized shells of two kinds of invertebrates are widely used to identify Permian strata and correlate them between sites: fusulinids, a kind of shelled amoeba-like protist that is one of the foraminiferans, and ammonoids, shelled cephalopods that are distant relatives of the modern nautilus. Terrestrial life in the Permian included diverse plants, fungi, arthropods, and various types of tetrapods. The Permian began with the Carboniferous flora still flourishing. About the middle of the Permian there was a major transition in vegetation. The swamp-loving lycopod trees of the Carboniferous, such as Lepidodendron and Sigillaria, were replaced by the more advanced conifers, which were better adapted to the changing climatic conditions. Lycopods and swamp forests still dominated the South China continent because it was an isolated continent and it sat near or at the equator. Oxygen levels were probably high there. The Permian saw the radiation of many important conifer groups, including the ancestors of many present-day families. The ginkgos and cycads also appeared during this period. Rich forests were present in many areas, with a diverse mix of plant groups. A number of important new insect groups appeared at this time, including the Coleoptera (beetles) and Diptera (flies). Permian tetrapods consisted of temnospondyli, lepospondyli and batrachosaur amphibians and sauropsids and synapsid (pelycosaurs and therapsids) reptiles. This period saw the development of a fully terrestrial fauna and the appearance of the first large herbivores and carnivores. Early Permian terrestrial faunas were dominated by pelycosaurs and amphibians, the middle Permian by primitive therapsids such as the dinocephalia, and the late Permian by more advanced therapsids such as gorgonopsians and dicynodonts. Towards the very end of the Permian the first archosaurs appeared (proterosuchid thecodonts); during the following, Triassic, period these latter would evolve into more advanced types, eventually into dinosaurs. Also appearing at the end of the Permian were the first cynodonts, which would go on to evolve into mammals during the Triassic. Another group of therapsids, the therocephalians (such as Trochosaurus), arose in the Middle Permian. Permian-Triassic extinction event The Permian ended with the most extensive extinction event recorded in paleontology: the Permian-Triassic extinction event. 90% to 95% of marine species became extinct, as well as 70% of all terrestrial organisms. On an individual level, perhaps as many as 99.5% of separate organisms died as a result of the event. There is also significant evidence that massive flood basalts from magma output contributed to environmental stress leading to mass extinction. The reduced coastal habitat and highly increased aridity probably also contributed. Another hypothesis involves ocean venting of hydrogen sulfide gas. Portions of deep ocean will periodically lose all of its dissolved oxygen allowing bacteria that live without oxygen to flourish and produce hydrogen sulfide gas. If enough hydrogen sulfide accumulates in an anoxic zone, the gas can rise into the atmosphere. Oxidizing gasses in the atmosphere would destroy the toxic gas but the hydrogen sulfide would soon consume all of the atmospheric gas available to convert it. Hydrogen sulfide levels would increase dramatically over a few hundred years. Modeling of such an event indicate that the gas would destroy ozone in the upper atmosphere allowing ultraviolet radiation to kill off species that had survived the toxic gas (Kump, et al, 2005). Of course, there are species that can metabolize hydrogen sulfide. An even more speculative hypothesis is that intense radiation from a nearby supernova was responsible for the extinctions. Trilobites, which had thrived since Cambrian times, finally became extinct before the end of the Permian. In 2006, a group of American scientists from the Ohio State University reported evidence for a possible huge meteorite crater (Wilkes Land crater) with a diameter of around 500 kilometers in Antarctica. The crater is located at a depth of 1.6 kilometers beneath the ice of Wilkes Land in eastern Antarctica. The scientists speculate that this impact may have caused the Permian-Triassic extinction event, although its age is bracketed only between 100 million and 500 million years ago. They also speculate that it may have contributed in some way to the separation of Australia from the Antarctic landmass, which were both part of a supercontinent called Gondwana. See also * Permian tetrapods References * Ogg, Jim; June, 2004, Overview of Global Boundary Stratotype Sections and Points (GSSP's) http://www.stratigraphy.org/gssp.htm Accessed April 30, 2006. * }} External links *University of California offers a more modern Permian stratigraphy *Classic Permian strata in the Glass Mountains of the Permian Basin * Category:Permian